The field of the invention is optical systems for use with laser devices. More specifically, the invention relates to optical systems designed to obtain anamorphic magnification with a substantially reduced focal length with minimal distortion or light loss.
It is often desirable to make non-contact measurements of the dimensions of an object. For example, a manufactured part can be inspected for tolerances at high speed. Single point range sensors are often used for this purpose and the most common such sensors use a light source and optical triangulation to determine range. An illumination source projects a spot of light onto the surface to be measured and receiving optics form an image of this spot on a light sensing detector. As the distance from sensor to surface changes, the position of the reflected spot image on the detector plane shifts. The lateral shift of position of the spot image on the detector can be used to meausre the distance between the sensor and the surface.
Typically, point range sensors are required to be small, accurate and inexpensive, and to have a large standoff distance between the sensor and the surface being measured. Simultaneously satisfying all of these requirements has led to a number of difficulties. A technique which overcomes many of these hurdles and may prove applicable to a wide range of other optical problems is presented herein. It is a method for obtaining high anamorphic magnification (i.e., different magnification of the image in each of two orthogonal directions) in a small space, for low cost.
In order to obtain high accuracy, the detector must be able to resolve small lateral shifts in the spot position. This generally requires high magnification in the direction of spot travel. When this requirement is combined with the common requirement that the sensor be separated from the object being measured by a large standoff distance two problems arise. First, since the image distance is the product of the magnification and the object distance, the resulting sensor package is quite large. Second, the high magnification leads to large spot image size and hence to low light levels on the detector since the available light is spread over a larger area.
There are several possible solutions to the first of these two problems. One is to fold the optical path of the sensor using mirrors, thereby shortening the sensor package. This decreases package length at the expense of increased package width. A second method is to use multiple lenses in the imaging system to produce a telephoto lens, reducing the distance from the lens assembly to the image plane. Third, one can also use two lenses as follows: the first lens acts as a relay, producing an image of unit (or less) magnification, and the second lens, free of the stand-off distance requirement, produces a magnified image. These solutions do not address the light level problem however, and thus are not entirely satisfactory.
The second problem can be tackled by increasing the lens diameter (increasing the size of the sensor package) or by using anamorphic optics to produce the required magnification in the direction of spot travel, while providing unit (or less) magnification in the orthogonal direction, thereby decreasing the area of the spot image relative to the non-anamorphic system, hence increasing the light level on the detector. Other solutions such as increasing the light level of the illumination source or the sensitivity of the detector are also possible.
It should be noted that no combination of the above partial solutions leads to an acceptable full solution. Increased package size is never desirable, and multi-element anamorphic optical elements can become expensive. Nor are holographic optical elements useful since their operation is wavelength dependent and the illumination source is usually a laser diode with considerable wavelength drift. What is needed is an apparatus or technique that solves both of the above problems simultaneously, while leaving cost and package size small.